The database manages most important pieces of data in an information system of an enterprise. Therefore, even if a part of hardware included in the database gets into a fault, a stable operation at a high level is required of the database. In a database system on which such a critical requirement is imposed, generally server-redundancy based on a cluster configuration has hitherto been implemented. The cluster configuration represents a system for providing services to users by combining a plurality of computers (called nodes). In recent years, however, in this type of system, the following problems have arisen.
Firstly, there might occur a case in which the operation stops due to a fault of a shared component such as a storage.
Secondly, when one of the nodes configuring the cluster is switched over to another node, switching time entails a long period of time, e.g., several tens of seconds through several minutes or longer, depending on a scale.
A most effective measure for coping with the problems given above is a perfect duplex configuration having none of the shared component (such as a shared disk in the conventional cluster configuration). In this case, a quick resumption of the operation is actualized by switching over the node at a high speed in a way that sets a standby node to stand by in a status where a mirroring environment of the database is always started up. This type of perfect duplex system is referred to as simply a duplex system.
The general type of duplex system manages the database in a way that duplexes the database into a main system and a subsystem. The main system and the subsystem exchange the information via a data transfer path. Then, one of the duplexed databases is set as the main system, and only the main system can be updated from an application. With this configuration, consistency between the databases in the duplex system is assured. Moreover, in the duplex system, a server executing the application (which will hereinafter be referred to as an application server) is not aware of the main system and the subsystem. Accordingly, the duplex system provides a function of automatically connecting the main database to the application server.
Moreover, the duplex system accumulates logs in the main system in preparation against a temporary fault on the data transfer path or in preparation for maintaining the subsystem. Then, there is provided a function of automatically restoring the duplex system after being restored from the fault on the data transfer path or after completion of maintaining the subsystem.
Further, the perfect duplex system such as the Duplex System performs, even when the fault occurs in the main-node, switchover to the sub-node, thereby enabling the operation to continue and availability to be improved. The perfect duplex system normally, if the data transfer path becomes abnormal due to abnormality in a network, however, exits the duplexing process because of being impossible of executing the duplexing process. Further, in the case of exiting the duplexing process, the data after exiting the duplexing process is required to be copied to the subsystem from the main system in order to restore again the duplexing process. Therefore, a load on the system when restored rises.
Such being the case, what is demanded as a system is that if there is a possibility of easily obviating the fault such as the abnormality in the network, the logs are accumulated in the main system, and, after obviating the fault, the duplex configuration is restored by automatically retransmitting the logs.    [Patent document 1] Japanese Patent Publication No. 2904100    [Patent document 2] Japanese Patent Application Laid-Open Publication No. 2001-022627